The analysis of particular lethal mutations has demonstrated the existence of neoplasms in Drosophila melanogaster (Gateff, 1978). The availability of temperature-senitive mutations provides a system in which the initiation of the neoplastic transformation can be controlled and analyzed. We have isolated and begun the characterization of temperature-sensitive alleles at two neoplastic loci, Tum 1 and 1(2)gl. The Tum 1 mutation expresses a neoplasm of the larval lymph gland and the 1(2)gl t.s. mutations express a neoplasm of the wing imaginal disc. The proposed research on the Tum 1 mutation involves the characterization of the cell types produced by this neoplastic mutation, the functions expressed by the transformed cells, and the interaction of the Tum 1 mutation with other mutations that express neoplasms of the larval lymph gland. The proposed research on the 1(2)gl mutations involves defining the effect of the initiation of the neoplastic transformation on: 1) The determines states of the wing disc cells; 2) The ability of the wing discs to undergo regeneration; 3) The gross morphology and fine structure of the wing discs, and 4) The onset of rapid uncontrolled growth. The analysis of these two mutants will provide a unique insight into the cellular changes that occur during the initiation of the neoplastic transformation.